Page 558 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

546 FLUID THERAPY

hypoalbuminemia presents additional considerations. If the patient remains hypotensive and there are concerns
An increase in the intravascular volume and hydrostatic about volume overload, central venous pressure monitor-
pressure from crystalloid infusion is not opposed by col- ing may be helpful. A CVP less than 0 cm H 2 O indicates
loid oncotic pressure in the plasma, enhancing interstitial hypovolemia, whereas a CVP more than 10 cm H 2 Ois a
edema in the periphery. Even with administration of a contraindication to further fluid therapy. A 10 to 15 mL/
colloidal solution, aggressive diuresis with a crystalloid kg bolus of crystalloid or 3 to 5 mL/kg of colloid will not
may not be possible without creating peripheral edema. change the CVP in hypovolemic patients, but will tran-
Loss of antithrombin III in urine causes a hypercoagula- siently increase the CVP by 2 to 4 cm H 2 O in the
ble state, which may cause complications associated with euvolemic patient, and cause a rise of more than 4 cm
intravenous catheterization. H 2 O in the hypervolemic patient. 62 Adequate resuscita-
Anemia may be present in both acute and chronic renal tion as assessed by achievement of identifiable goals
failure. Red cell survival is shorter in the uremic environ- decreases renal morbidity as compared with using stan-
ment, blood sampling may create substantial losses, and dard resuscitation doses in people. 30
erythropoietin production is generally suppressed. Gas- For patients with dehydration, the dehydration deficit
trointestinal bleeding can acutely cause anemia, and if is calculated as body weight (in kilograms) estimated %
bleeding is brisk, hypotension and hypovolemia may dehydration ¼ fluid deficit in L. Because dehydration of
occur and require rapid infusion of crystalloid or synthetic less than 5% cannot be detected by clinical examination, a
colloid solutions. Red blood cell transfusion may be 5% dehydration deficit is presumed in patients with AKI
indicated if symptomatic anemia is present. Intensive that appear normally hydrated. If a fluid bolus was used
diuresis may exacerbate high output heart failure in cats for initial resuscitation, that volume is subtracted from
with anemia. Conversely, rapid blood transfusion may the dehydration deficit.
cause congestive heart failure. In patients with The rate of replacing the dehydration deficit depends
compromised cardiovascular function or patients with on the clinical situation. In patients with AKI, who have
incipient volume overload, red cell transfusion may need presumptively become dehydrated over a short period of
to be given more slowly than usual. time, rapid replacement is prudent. This restores renal
A sometimes overlooked fluid choice is water given perfusion to normal levels and may help prevent further
enterally. Because vomiting is a common problem with damage to the kidneys. In situations where urine output
uremia, enteral food or water is frequently may be diminished, rapid replacement of dehydration
contraindicated, and many uremic patients will not vol- deficits to normalize the fluid status allows the clinician
untarily consume water. However, water administered to quickly determine if oliguria is an appropriate response
through a feeding tube should be included in water to volume depletion or is a pathologic change from the
calculations. renal failure. In that setting, replacing the deficit in 2 to
Ultimately, the fluid choice must be guided by moni- 4 hours is recommended. If there is potential compro-
toring the patient’s fluid and electrolyte balance. A major mise of diastolic function of the heart, a rapid fluid bolus
determining factor in the appropriate fluid choice is the may precipitate congestive heart failure, and a more grad-
sodium concentration because the degree of free-water ual rehydration rate (i.e., over 12 to 24 hours) may be
loss relative to sodium loss varies greatly in patients with prudent.
AKI. The guiding principal in treating a sodium disorder In patients with chronic dehydration, a more gradual
is to reverse it at the same rate at which it developed replacement of the fluid deficit is acceptable to minimize
because rapid increases or decreases in sodium concentra- the risk of cardiac problems or too rapid changes in
tion may cause CNS dysfunction (see next section). electrolytes, and 24 hours is a commonly selected time
frame. In severely dehydrated, chronic debilitated
Volume and Rate patients, it may take up to 48 hours to rehydrate.
Some patients may present in hypovolemic shock, which The concept of maintenance fluid rate is based on aver-
is manifest as dull mentation, hypotension (systolic blood age fluid losses from insensible (respiration) and sensible
pressure <80 mm Hg), poor perfusion of the periphery (urine output) sources. There are a variety of published
(cold extremities, pale/gray mucous membranes with values. The most commonly quoted value is 66 mL/
slow capillary refill time), hypothermia, or tachycardia. 62 kg/day. Ignoring normal individual variation, the pre-
Immediate correction of shock is necessary to prevent fur- sumption with this value is that urine output is normal
ther and irreversible organ damage. The standard dose of and there are no other sources of fluid loss, which is rarely
crystalloids is 60 to 90 mL/kg for dogs and 45 to the case in patients with renal failure. However, it makes a
60 mL/kg for cats, of which one fourth is given over 5 reasonable starting point for calculating fluid administra-
to 15 minutes. 41 If hemodynamic parameters do not tion volumes. If accurate measurement of urine output
improve sufficiently with the first one fourth dose, a sec- and ongoing losses is available, fluid therapy can be
ond dose should be given. Resuscitation efforts are adjusted precisely (see “ins-and-outs” method below).
continued until the patient is hemodynamically sound. If these parameters are not accurately measured, an